In numerous localities, substantial quantities of wood and wood scraps are available which may provide an inexpensive source of fuel. A problem has always existed concerning the burning of wood efficiently for heating purposes, and particularly in burning green wood or wood which has not properly dried or been seasoned.
One of the most important problems in regard to burning of wood for space heating purposes relates to the fact that substantially all wood and especially green wood contains a very substantial amount of creosote and moisture along with some volatile gases such as methane. In the burning of wood, the creosote and moisture is driven off the wood by the heat, and in most wood burning devices, oxygen and moisture cause the creosote to solidify on the interior surfaces of the heat exchanger and stack or flue. As time goes by, the quantity of solidified creosote builds up on the interior of the heat exchanger or stack so that less and less heat is transmitted to the space being heated, and a greater and greater risk of chimney fires is created.
The woods which are readily available and inexpensive for use in heating are those which are high in creosote content, and such woods include poplar or aspen, willow, pine, tamarack, cedar, balsam, hemlock and birch.
In some localities sawmill slabs or scrap is readily available and inexpensively obtained, but such material is primarily the outer parts of logs including bark, which is the particular part where a substantial amount of creosotes are found.
In the past, there have been some furnaces or burners which have attempted to burn wood or alternately burn oil in the same firebox or combustion chamber. However, it does not produce efficient burning of the wood or green wood by simply burning oil in the same combustion chamber.